1. Field of the Invention
The invention relates generally to a ball screw unit.
2. Description of the Related Art
A ball screw unit that includes a screw shaft and a ball screw nut that is screwed onto the screw shaft via balls is commonly used for an electric actuator or a shock absorber. For example, Japanese Patent Application Publication No. 2005-264992 (JP-A-2005-264992) describes that a ball screw unit, which has a ball screw nut that is connected to a motor and a screw shaft that moves linearly, is applied to an electromagnetic shock absorber.
The electromagnetic shock absorber converts axial vibrations from a tire into rotational vibrations with the use of the ball screw unit, and controls a damping force with the use of the motor. When overstroke of a suspension occurs, for example, because a vehicle runs on to a bump, the motor, which has been rotating at a high speed, may stop suddenly and an excessive axial force may be applied to a ball screw due to an inertia torque. How to protect a ball screw mechanism in this case is an issue that needs to be resolved. In order to ensure adequate strength of the ball screw, a larger-sized ball screw unit may be employed. However, employing a larger-sized ball screw unit causes some inconveniences. For example, an inertia moment increases, which creates a need for a larger-sized motor. As a result, response of the motor deteriorates.